This invention relates to air bag systems for use in automotive vehicles and more particularly to an AC firing circuit for triggering various types of air bag systems.
It is known in the art relating to vehicle air bag systems to provide a system that includes a firing circuit that applies energy to a firing element, or squib, to cause inflation of an air bag. The firing circuit is controlled by a deployment command signal which is sent by a microprocessor when the microprocessor has determined that a crash situation exists requiring deployment of the air bag.
A DC or AC firing circuit may be used to trigger the firing element. One example of an AC firing circuit includes a capacitor coupled in series with the firing element. The capacitor has a relatively small capacitance value such that a direct voltage applied to the firing element would be insufficient to fire the firing element. Only after a series of AC current pulses will sufficient energy be transferred to the firing element to cause deployment of the air bag.
One disadvantage of such an AC firing circuit is that it requires a tuned frequency to supply maximum energy to the firing element. Variations in the value of inductance result in different levels of energy being transferred to the firing element. Wiring harnesses connecting various air bags within the vehicle to a firing circuit have different values of inductance depending on the harness length and its routing. Also, whether the air bag requires a clockspring will affect the value of inductance. Thus, it is desirable to have a firing circuit that supplies maximum energy to the firing element over a wide range of inductance.
The present invention provides a method and universal firing system for providing sufficient energy to trigger a firing element causing deployment of various types of air bags within a vehicle.
An object of the present invention is to provide an AC firing circuit for supplying maximum energy to trigger a firing element to cause deployment of various types of air bags over a wide range of inductance.
Another object of the present invention is to provide the ability to trigger the firing element even if there is a short to ground or the power supply within the firing circuit.
The universal firing system of the present invention includes an acceleration sensor for producing an output signal indicative of the vehicle""s acceleration. From the output signal of the acceleration sensor, an air bag controller determines whether a crash condition exists requiring deployment of an air bag. If deployment of the air bag is necessary, the controller sends a firing command signal to a power switching circuit which supplies charge and discharge current pulses of alternating polarity to a firing loop having a firing element in series with an unknown inductance and a capacitor. In order to provide maximum current to the firing element, a delay circuit is connected to the power switching circuit to control and adjust the frequency of the charge and discharge current pulses applied by the power switching circuit to the resonant frequency of the firing loop.
The delay circuit detects a charge or discharge period of the capacitor and delays the charge or discharge current pulse of the series for a delay period proportional to the detected charge or discharge period. Then, the delay circuit drives the power switching circuit to apply a next current pulse of the series in an opposite direction of the present current pulse. After the series of current pulses, a sufficient amount of energy is transferred to the firing element causing a chemical reaction which generates a gas and causes the air bag to inflate.
The method of the present invention includes the steps of receiving a firing command signal from an air bag controller indicative of the existence of a crash condition requiring deployment of an air bag. In response to the fifing command signal, a power switching circuit applies a series of charge and discharge current pulses of alternating polarity to a firing loop having a firing element coupled in series with unknown inductance and a capacitor. During the application of the current pulses, the charging and discharging of the capacitor to a predetermined low voltage level is detected and measured. This measured charge or discharge period, which provides an indication of inductance, is used to calculate a delay period for delaying the start of the next current pulse in the series to adjust the frequency of the current pulses to the resonant frequency of the firing loop. Thus, by maintaining the current at resonant frequency, maximum current is provided to the firing element.
These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings.